1. Technical Field
The present invention relates generally to gimbal-based tracking systems, and more particularly to a software-based resolver-to-digital converter implemented with existing system hardware, thereby reducing system size and cost.
2. Discussion
In many gimbal-based tracking and surveillance systems, it is necessary to determine the angle of rotation of a gimbaled instrument with respect to its mount in order for the system to function accurately. For example, in a two axis azimuth-elevation gimbal system, the system must track both the position of the inner elevation gimbal with respect to the outer azimuth gimbal, and the position of the outer azimuth gimbal with respect to the base. In order to compute the relative positions of the elevation and azimuth gimbals, a device must measure relative angles between the two gimbals.
A resolver is a common angular measurement device used for measuring these relative angles between the elevation and azimuth gimbals. A resolver exhibits excellent performance, is relatively low in cost, is small in size, and exhibits a high degree of reliability. Because of these characteristics, a resolver is ideal for use in precision pointing and stabilization systems.
In spite of the above-mentioned desirable characteristics, a resolver suffers from a limitation: it does not output the gimbal angle of rotation, but rather the sine and cosine of the particular angle of rotation. Thus, external instrumentation is required to compute the angle from the resolver-generated sine and cosine measurements.
The most common device for computing the angle of rotation from the resolver output is the resolver-to-digital converter integrated circuit (RDC). An RDC receives the sine and cosine analog outputs from the resolver, together with an analog reference signal, and produces a digital output that represents the angle of rotation.
Commercially available RDCs exhibit several desirable features. First, an RDC produces a digital output. As gimbal systems increasingly employ digital control electronics, the digital output from the RDC functions as an analog-to-digital interface with these digital control electronics. Second, an RDC is flexible in that the RDC is usually compatible with many types of resolvers, including resolvers with different reference frequencies. Third, an RDC is usually tolerant of reference amplitude and frequency drift, along with resolver temperature changes. Fourth, an RDC is relatively fast and produces a digital angle of rotation with relatively small time delay.
However, presently commercial RDCs also have several limitations. First, the cost of a typical RDC integrated circuit is high compared to other integrated circuits. As a result, the cost of a digital controller card using one or more RDCs is greatly increased. Second, an RDC consumes a considerable amount of physical space on a circuit board on which it is implemented. Typically, an RDC integrated circuit chip may range from 28 pins to as many as 44 pins. As increased emphasis is continually placed on reducing the number of circuit boards needed to implement control system electronics, the size of the RDC is becoming more and more of a critical factor. Third, commercially available RDCs are inflexible in that the RDCs can not be tailored to meet program-specific requirements. For example, the maximum reference frequency that can be handled by current generation RDCs is approximately 25 KHz. If a custom built resolver had a reference frequency of 40 KHz, a current generation RDC would be unable to determine the resolver angle. Thus, a custom made part would have to be manufactured, driving up the cost of the system considerably.
What is needed then is a resolver-to-digital converter that exhibits all of the above-mentioned advantages associated with commercially available RDCs and that is relatively inexpensive to implement, requires a relatively small amount of circuit board space and is flexible so that it is capable of meeting program-specific requirements.
In accordance with the teachings of the present invention, a software-based resolver-to-digital converter (RDC) is provided for use in computing the angle of rotation between gimbaled instruments in a gimbal-based system. The software-based RDC finds particular utility in military and airline surveillance, targeting and tracking systems in which position transducers such as resolvers are used to track the rotation of gimbal-mounted instrumentation. The present invention utilizes existing system hardware programmed to perform requisite computations. As a result, system cost and size is reduced.
In the inventive approach, gimbal-mounted instrumentation is provided in a gimbal system. A resolver is used to measure parameters of an angle of rotation associated with the instrumentation as the instrumentation rotates on the gimbal. The resolver outputs the measured parameters as analog signals, along with an analog reference signal. Multiplexing means are used to multiplex the signals output from the resolver. An analog to digital converter then converts the multiplexed analog signals to digital signals.
Digital processing means is programmed to compute the angle of rotation. The digital processing means includes three main components: filter means for receiving the multiplexed digital signals and for reducing unwanted noise and signal sensitivity; computing means for computing a value for the angle of rotation from the filter signals; and output means for outputting the computer angle of rotation to the system for processing to enhance system operation. The filter means and the computing and output means are coded into and implemented at the existing digital processing means, thereby eliminating the need for additional resolver-to-digital converter hardware components and reducing the cost of the system.